Serotonin (5-hydroxtyptmine, 5-HT) mediates a wide range of physiological functions by activating multiple receptors. Most 5-HT receptors regulate intracellular signal transduction system through GTP-binding proteins. These intracellular signal transductions mediate by 5-HT receptors of somato-dendritic region of neurons were mostly studied using brain slice preparations. Though 5-HT receptors distribute not only at the somato-dendritic region but also at the postsynaptic region, physiological properties of the postsynaptic 5-HT receptors remain nuclear. We attempted to analyze the action of 5-HT on synaptic transmission using autapse of the rat hippocampal neuron in culture, as both pre- and postsynaptic action could be analyzed in a single cell. The action of 8-OH-DPAT, felsinoxan and fluvoxamine on the epsp induced in the CA1 hippocampal region was also studied using paired-pulse facilitation method. We found that 5-HT suppressed the amplitude of excitatory postsynaptic current and this action was suggested to be presynaptic action and mediated through 5-HT1A receptor.